Conventionally, there has been widely known an air-fuel ratio control apparatus, which includes a three-way catalyst (53) disposed in an exhaust passage of an internal combustion engine, and an upstream air-fuel ratio sensor (67) and a downstream air-fuel ratio sensor (68) disposed in the exhaust passage so as to be located upstream and downstream, respectively, of the three-way catalyst, as shown in FIG. 1.
This air-fuel ratio control apparatus calculates an “air-fuel ratio feedback amount (quantity) to have an air-fuel ratio of an air-fuel mixture supplied to the engine (air-fuel ratio of the engine) coincide with the stoichiometric air-fuel ratio” based on the outputs of the upstream and downstream air-fuel ratio sensors, and feedback-controls the air-fuel ratio of the engine based on the air-fuel ratio feedback amount such that the air-fuel ratio of the engine coincides with the stoichiometric air-fuel ratio.
Furthermore, there has been widely known an air-fuel ratio control apparatus which has the upstream air-fuel ratio sensor but does not have the downstream air-fuel ratio sensor. The air-fuel ratio control apparatus calculates an “air-fuel ratio feedback amount to have the air-fuel ratio of the engine coincide with the stoichiometric air-fuel ratio” based solely on the output of the upstream air-fuel ratio sensor, and feedback-controls the air-fuel ratio of the engine based on the air-fuel ratio feedback amount.
The air-fuel ratio feedback amount used in each of those air-fuel ratio control apparatuses is a control amount commonly used for all of the cylinders.
Incidentally, in general, such an air-fuel ratio control apparatus is applied to an internal combustion engine using an electronic-control-fuel-injection apparatus. The internal combustion engine has at least one fuel injection valve (39) at each of cylinders or at each of intake ports communicating with the respective cylinders. Accordingly, when the characteristic/property of the fuel injection valve of a certain (specific) cylinder changes so as to inject fuel in an amount excessively larger than an injection amount to be injected according to an instruction (instructed fuel injection amount), only an air-fuel ratio of an air-fuel mixture supplied to that certain cylinder (the air-fuel ratio of the certain cylinder) greatly changes toward the rich side. That is, the degree of air-fuel ratio non-uniformity among the cylinders (inter-cylinder air-fuel ratio variation; inter-cylinder air-fuel ratio imbalance) increases. In other words, there arises an imbalance among “cylinder-by-cylinder air-fuel ratios”, each of which is the air-fuel ratio of the air-fuel mixture supplied to each of the cylinders.
It should be noted that a cylinder corresponding to the fuel injection valve having the characteristic to inject the fuel in an amount excessively larger or excessively smaller than the instructed fuel injection amount is also referred to as an imbalanced cylinder, and each of the remaining cylinders (a cylinder corresponding to the fuel injection valve having the characteristic to inject the fuel in an amount equal to the instructed fuel injection amount) is also referred to as an un-imbalanced cylinder (or a normal cylinder).
In the case described above, an average of the air-fuel ratios of the air-fuel mixtures supplied to the entire engine becomes an air-fuel ratio in the rich side in relation to (with respect to) the stoichiometric air-fuel ratio. Accordingly, by means of the air-fuel ratio feedback amount commonly used for all of the cylinders, the air-fuel ratio of the above-mentioned certain cylinder is changed toward the lean side so as to come closer to the stoichiometric air-fuel ratio, and, at the same time, the air-fuel ratios of the remaining cylinders are changed toward the lean side so as to deviate more greatly from the stoichiometric air-fuel ratio. As a result, the average of the air-fuel ratios of the air-fuel mixtures supplied to the entire engine becomes equal to the stoichiometric air-fuel ratio.
However, since the air-fuel ratio of the certain cylinder is still in the rich side in relation to the stoichiometric air-fuel ratio and the air-fuel ratios of the remaining cylinders are in the lean side in relation to the stoichiometric air-fuel ratio, combustion of the air-fuel mixture in each of the cylinders fails to become complete combustion. As a result, the amount of emissions (the amount of unburned combustibles and/or the amount of nitrogen oxides) discharged from each of the cylinders increase. Therefore, even when the average of the air-fuel ratio of the air-fuel mixture supplied to the engine is equal to the stoichiometric air-fuel ratio, the increased emissions cannot be removed by the three-way catalyst. Consequently, the amount of emissions may increase.
Accordingly, in order to prevent the emissions from increasing, it is important to detect a state in which the degree of air-fuel ratio non-uniformity among the cylinders becomes excessively large (generation of an inter-cylinder air-fuel ratio imbalance state) and take some measures against the imbalance state. It should be noted that, the inter-cylinder air-fuel ratio imbalance also occurs, for example, in a case where the characteristic of the fuel injection valve of a certain cylinder changes to inject fuel in an amount excessively smaller than the instructed fuel injection amount.
One of conventional fuel injection amount control apparatuses obtains a trace/trajectory length of the output value (output signal) of the upstream air-fuel ratio sensor (67). Further, the control apparatus compares the trace length with a “reference value which changes in accordance with the rotational speed of the engine”, and determines whether or not the inter-cylinder air-fuel ratio imbalance state has occurred based on the result of the comparison (see, for example, U.S. Pat. No. 7,152,594).
Another conventional fuel injection amount control apparatus analyzes the output value of the upstream air-fuel ratio sensor (67) so as to detect the cylinder-by-cylinder air-fuel ratios. Further, the control apparatus determines whether or not the inter-cylinder air-fuel ratio imbalance state has occurred, based on a difference between the detected cylinder-by-cylinder air-fuel ratios (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2000-220439).
Meanwhile, when the non-uniformity among the cylinder-by-cylinder air-fuel ratios occurs, there may be a case in which a true average (true temporal average) of the air-fuel ratio of the air-fuel mixture supplied to the entire engine is controlled so as to become an air-fuel ratio larger (leaner) than the stoichiometric air-fuel ratio by means of the air-fuel ratio control amount. The reason for this will next be described.
The fuel supplied to the engine is a chemical compound of carbon and hydrogen. Accordingly, “carbon hydride HC, carbon monoxide CO, hydrogen H2, and so on” are generated as intermediate products, when the air-fuel ratio of the mixture to be combusted is richer than the stoichiometric air-fuel ratio. In this case, as the air-fuel ratio of the mixture for the combustion becomes richer in relation to the stoichiometric air-fuel ratio and deviates more greatly from the stoichiometric air-fuel ratio, a probability that the intermediate products meet and bind to the oxygen molecules during the combustion becomes drastically smaller. Consequently, as shown in FIG. 2, an amount of the unburnt substances (HC, CO, and H2) drastically (e.g., in a quadratic function fashion) increases, as the air-fuel ratio of the mixture supplied to the cylinder becomes richer.
It should be noted that the abscissa axis of the graph shown in FIG. 2 is an “imbalance ratio (rate).” The imbalance ratio means a “ratio (Y/X) of a difference Y (=(X−af)) between the stoichiometric air-fuel ratio X and the air-fuel ratio of the imbalanced cylinder to the stoichiometric air-fuel ratio X.
It is now assumed that only of the air-fuel ratio of the certain cylinder deviates greatly toward the rich side. Under this assumption, the air-fuel ratio (air-fuel ratio of the certain cylinder) of the air-fuel mixture supplied to the certain cylinder changes to a rich (small) air-fuel ratio in a great amount, compared to the air-fuel ratios (air-fuel ratios of the remaining cylinders) of the air-fuel mixtures supplied to the remaining cylinders. At this time, a great amount of unburnt substances (HC, CO, and H2) are discharged from that certain cylinder.
In the mean time, the upstream air-fuel ratio sensor (67) generally comprises a diffusion resistance layer. The upstream air-fuel ratio sensor (67) outputs a value corresponding an amount of oxygen or unburnt substance that has reached an exhaust-gas-side electrode layer (surface of an air-fuel ratio detection element) of the upstream air-fuel ratio sensor (67) after passing through the diffusion resistance layer.
Further, hydrogen H2 is a small molecule, compared with carbon hydride HC, carbon monoxide CO, and the like. Accordingly, hydrogen H2 rapidly diffuses through the diffusion resistance layer of the upstream air-fuel ratio sensor (67), compared to the other unburnt substances (HC, CO). Therefore, when a large amount of the unburnt substances including HC, CO, and H2 are generated, a preferential diffusion of hydrogen H2 occurs in the diffusion resistance layer. That is, hydrogen H2 reaches the exhaust-gas-side electrode layer in a larger amount, compared with the “other unburnt substances (HC, CO)”.
As a result, a fraction of hydrogen H2 to all of the unburnt substances included in the “exhaust gas reaching the exhaust-gas-side electrode layer of the upstream air-fuel ratio sensor” becomes greater than a fraction of hydrogen H2 to all of the unburnt substances included in the “exhaust gas discharged from the engine.”
Due to the preferential diffusion of hydrogen when the non-uniformity among cylinder-by-cylinder air-fuel ratios is occurring, an air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor (67) becomes richer than an average of a true air-fuel ratio of the mixture supplied to the entire engine (average of a true air-fuel ratio of the exhaust gas discharged from the engine).
More specifically, for example, it is assumed that an air-fuel ratio A0/F0 is equal to the stoichiometric air-fuel ratio (e.g., 14.6), when the intake air amount (weight) introduced into each of the cylinders of the 4-cylinder engine is A0, and the fuel amount (weight) supplied to each of the cylinders is F0.
Under this assumption, it is further assumed that an amount of the fuel supplied (injected) to each of the cylinders becomes uniformly excessive in (or by) 10%. That is, it is assumed that the fuel of 1.1·F0 is supplied to each of the cylinders. Here, a total amount of the intake air supplied to the four cylinders (i.e., amount of intake air supplied to the entire engine during a period in which each and every cylinder completes one combustion stroke) is equal to 4·A0, and a total amount of the fuel supplied to the four cylinders (i.e., amount of fuel supplied to the entire engine during the period in which each and every cylinder completes one combustion stroke) is equal to 4.4·F0 (=1.1·F0+1.1·F0+1.1·F0+1.1·F0). Accordingly, a true average of the air-fuel ratio of the mixture supplied to the entire engine is equal to 4·A0/(4.4·F0)=A0/(1.1·F0).
At this time, the output value of the upstream air-fuel ratio sensor (67) becomes equal to an output value corresponding to the air-fuel ratio A0/(1.1·F0). Accordingly, the air-fuel ratio of the mixture supplied to the entire engine is caused to coincide with the “stoichiometric air-fuel ratio A0/F0 serving as a target air-fuel ratio”, by the feed-back control. In other words, the amount of the fuel supplied to each of the cylinders is decreased in (by) 10% by the air-fuel ratio feedback control. Therefore, the fuel of 1·F0 is supplied to the each of the cylinders, and thus, the air-fuel ratio of each of the cylinders becomes equal to the stoichiometric air-fuel ratio.
Next, it is assumed that an amount of the fuel supplied to one certain specific cylinder is excessive in (by) 40% (i.e., 1.4·F0), and an amount of the fuel supplied to each of the remaining three cylinders is equal to a fuel amount required to have each of the air-fuel ratios of the other three cylinders coincide with the stoichiometric air-fuel ratio (i.e., F0).
Under this assumption, a total amount of the air supplied to the four cylinders is equal to 4·A0. A total amount of the fuel supplied to the four cylinders is equal to 4.4·F0 (=1.4·F0+F0+F0+F0). Accordingly, the true average of the air-fuel ratio of the mixture supplied to the entire engine is equal to 4·A0/(4.4·F0)=A0/(1.1·F0). That is, the true average of the air-fuel ratio of the mixture supplied to the entire engine in this case is equal to the value obtained “when the amount of the fuel supplied to each of the cylinders is uniformly excessive in (by) 10%” as described above.
However, as described above, an amount of the unburnt substances (HC, CO, and H2) in the exhaust gas drastically increases, as the air-fuel ratio of the mixture supplied to the cylinder becomes richer. In addition, an exhaust gas that is a mixture of gas discharged from each of the cylinders reaches the upstream air-fuel ratio sensor (67). Accordingly, an “amount of hydrogen H2 included in the exhaust gas in the case in which only the amount of the fuel supplied to the certain cylinder becomes excessive in (by) 40%” becomes prominently greater than an “amount of hydrogen H2 included in the exhaust gas in the case in which the amount of the fuel supplied to each of the cylinders is uniformly excessive in (by) 10%.”
Consequently, due to the “preferential diffusion of hydrogen” described above, the air-fuel ratio represented by the output value of the upstream air-fuel ratio sensor (67) becomes an air-fuel ratio richer than the “true average (A0/(1.1·F0) of the air-fuel ratio of the mixture supplied to the entire engine.” That is, even when the average of the air-fuel ratio of the exhaust gas is a “certain air-fuel ratio in the rich side”, a concentration of hydrogen H2 reaching the exhaust-gas-side electrode layer of the upstream air-fuel ratio sensor (67) when the inter-cylinder air-fuel ratio imbalance (air-fuel ratio imbalance among cylinders) is occurring becomes prominently higher than that when the inter-cylinder air-fuel ratio imbalance is not occurring. Accordingly, the output value of the upstream air-fuel ratio sensor (67) becomes a value indicating an air-fuel ratio richer than the true average of the air-fuel ratio of the air-fuel mixture.
Consequently, by the feedback control based on the output value of the air-fuel ratio sensor, the true average of the air-fuel ratio of the mixture supplied to the entire engine is caused to be leaner than the stoichiometric air-fuel ratio. This is the reason why the true average of the air-fuel ratio is controlled to be in the lean side when the non-uniformity among cylinder-by-cylinder air-fuel ratios occurs. It should be noted that such a “deviation of the air-fuel ratio toward the lean side due to the preferential diffusion of hydrogen and the feedback control” is also referred to as a “shift of the air-fuel ratio toward the lean side due to the preferential diffusion of hydrogen.”
The “shift of the air-fuel ratio toward the lean side due to the preferential diffusion of hydrogen” also similarly occurs when the air-fuel ratio of the imbalanced cylinder becomes leaner than the air-fuel ratio of the un-imbalanced cylinder. The reason for this will be described later.
It should be noted that another conventional fuel injection amount control apparatus determines whether or not the inter-cylinder air-fuel ratio imbalance has occurred utilizing the above phenomena. That is, the control apparatus performs the feedback control (main feedback control) to have the air-fuel ratio of the engine coincide with the stoichiometric air-fuel ratio based on the output value of the upstream air-fuel ratio sensor (67). Further, the control apparatus performs a feedback control (sub feedback control) to have the output value of the downstream air-fuel ratio sensor (68) coincide with a target value corresponding to the stoichiometric air-fuel ratio.
Hydrogen H2 included in the exhaust gas discharged from the engine is oxidized (purified) together with the other unburnt substances (HC, CO) in the catalyst (53). The exhaust gas that has passed through the catalyst (53) reaches the downstream air-fuel ratio sensor (68). Accordingly, the output value of the downstream air-fuel ratio sensor (68) becomes a value corresponding to the true air-fuel ratio of the mixture supplied to the engine.
Consequently, when only of the air-fuel ratio of the certain cylinder deviates greatly toward the rich side, the output value of the downstream air-fuel ratio sensor becomes a value corresponding to the “true air-fuel ratio which has been excessively corrected to the lean side” due to the feedback control based on the output value of the upstream air-fuel ratio sensor (67). That is, output value of the downstream air-fuel ratio sensor (68) becomes a value which varies in accordance with the degree of the inter-cylinder air-fuel ratio imbalance, and therefore, a control amount (sub feedback amount) which is used in the feedback control to have the output value of the downstream air-fuel ratio sensor (68) coincide with the target value corresponding to the stoichiometric air-fuel ratio becomes a value reflecting (in accordance with) the degree of the inter-cylinder air-fuel ratio imbalance. In view of the above, the conventional control apparatus determines whether or not the inter-cylinder air-fuel ratio imbalance has occurred based on the control amount of the sub feedback control (e.g., refer to Japanese Patent Application Laid-Open (kokai) No. 2009-30455).